Automatic temperature control devices are well known in the art of temperature control. A temperature controller is an instrument used to control temperature. Many prior art temperature controllers have temperature sensors for determining the ambient temperature within a controlled volume. The temperature controller attempts to maintain a set-point or “ideal” temperature within the controlled volume by regulating the output from a control element such as a heater or fan. The temperature controller attempts to reach a steady state where the temperature of the controlled volume is consistent with the set-point temperature.
There are several basic types of temperature controllers. One type is an on-off controller. Others are the proportional and the proportional with integral and derivative control (PID). The on-off controller is the simplest form of temperature control devices. This type of controller simply maintains the control element in either an on or off state. Once the temperature rises above or falls below the set-point temperature, the controller changes state, switching the output. For example, when an on-off controller is used for cooling, the output is on when the temperature is above the set-point and off when the temperature is below the set-point. The temperature must rise above or fall below the set-point temperature before the output of the control element changes.
Complicated and expensive temperature controllers have an inherent problem in reaching the set-point temperature. As the temperature of the control volume nears the set-point temperature, the control element may stay on too long or turn off too soon, which will result in the condition called overshooting or undershooting. Overshooting results when the temperature rises above the set-point temperature and undershooting results when the temperature falls below the set-point temperature. This may result in a process of the temperature cycling continually, rising above or falling below the set-point temperature before a steady state is reached. Because there is no “middle state” between on and off, the controller rapidly switches the temperature control element off and on as the actual temperature hovers around the set-point temperature. This condition is known as “chattering.” Chattering may damage the heating or cooling elements in a temperature control system due to the increased duty-cycle on the devices as they are rapidly turned off and on.
To compensate for “chattering” in on-off controllers, manufactures design a “hysteresis” or temperature differential into the controller. A hysteresis or temperature differential is essentially a buffer or delay in the control circuit. This buffer is a temperature range near the set-point temperature in which the controller operates. The actual temperature must be out side this range before the controller changes state, turning the control element on or off again. Thus, the temperature differential prevents the controller from switching the control elements on and off rapidly around the set-point temperature. In prior art systems this differential may be designed in to the circuit, controller software or it may be inherent to the system, due to the characteristics of the electronics or materials used. Though they are typically inexpensive, these controllers are not usually used where precise temperature control is required.